Photonic crystals (PhCs) are periodic dielectric or metallic structures that exhibit frequency regions, called photonic band gaps (PGBs), in which electromagnetic waves cannot propagate. The interest in PhCs arises from the fact that photon behavior in a dielectric structure is similar to the behavior of electrons in a semiconductor. The periodic arrangement of atoms in a semiconductor lattice opens up forbidden gaps in the energy band diagram for the electrons. Similarly in all-dielectric PBG structures, the periodic placement of dielectric “atoms” opens up forbidden gaps in the photon energy bands.
The idea of PhCs has led to the proposal of many novel applications at optical wavelengths, such as energy-efficient light emitters, thresholdless lasers, single-mode light-emitting-diodes and optical wave guides. In addition, PhCs are already being used in the millimeter and microwave regimes, where the applications include efficient reflectors, antennas, filters, sources and wave guides. They have also found possible applications as infrared filters.
The PhCs behave as ideal reflectors in the band gap region. Depending on the directional periodicity of these dielectric structures, the band gap may exist in 1-D, 2-D or all the three directions. Various lattice geometries were studied to find a periodic structure that would exhibit a PBG in all the directions. After several unsuccessful attempts in finding the right lattice geometry using “trial-and-error” techniques, researchers at Iowa State University were first to predict the existence of complete PBGs in a periodic dielectric structure arranged in diamond lattice geometry. Diamond lattice structures were calculated to have large gaps for a refractive index ratio between the two dielectrics as low as two.
After the initial research into the existence of photonic band gaps, there was an increased effort to find structures that could be more easily fabricated. These fabrication techniques include creating the PhC through emulsions, with carbon structures, and by creating crystals by a liquid-phase chemical reaction to infiltrate a polystyrene template. Another fabrication technique is the use of microfabrication techniques since these techniques permit fabrication of three-dimensional (3D) devices with functionality not possible in planar devices. Traditional fabrication methods using photolithography are usually slow and costly and are a barrier to the commercialization of PhCs. Over the last few years, a number of approaches for fabricating 3D microstructures have been reported as alternatives to conventional photolithography such as microtransfer molding, two-photon polymerization, holographic lithography, and nanoimprinting. Among these approaches, microtransfer molding showed a number of advantages, including low cost, capability for non-periodic 3D structures, a wide range of materials compatibility, and flexibility in design.
In conventional microtransfer molding, a liquid prepolymer fills microchannels formed on the surface of an elastomeric mold. The prepolymer is solidified after bringing the mold into contact with a substrate. Then, the structure is transferred to the substrate by removing the flexible mold. In spite of the many advantages of conventional microtransfer molding, uncured filled prepolymer can smear out of the channels by capillary wicking when contacting a substrate. This wicking deteriorates structural fidelity and requires an additional processing step to remove the uncontrolled polymer, such as reactive-ion etching. Partial curing of filled prepolymer is effective in avoiding capillary wicking in the elastomeric mold since the partial curing increases the viscosity of the prepolymer; however, uncured prepolymer is still favored to ensure sufficient bonding strength with the substrate and/or other layers.